1. Field
Embodiments of the present invention relate to interferometers and, more particularly, to an interferometer for measuring a surface shape of an examined surface or a transmitted wavefront through an examined optical system.
2. Description of the Related Art
In conventional interferometers, a light source and an examined object (examined surface or examined optical system) are in a nonconjugate relation. For example, a point light source is used in the interferometers of almost all types including the Fizeau interferometer and the Twyman-Green interferometer, and the point light source and the examined object are not optically conjugate with each other. Furthermore, an area light source is used in the Michelson interferometer, and the area light source and the examined object are not conjugate with each other.
The conventional interferometers are limited in the range of radii of curvature of examined surfaces available for measurement by a single interferometer. In the case of the conventional interferometers, if an examined surface and a reference surface have a large difference in their surface shape, the pitch of interference fringes will become too small to measure. In order to improve the contrast of interference fringes to be detected by a sensor like a CCD, there is a proposal of the Sub-Nyquist measurement method to reduce apertures of light receiving elements of a CCD, but it is unsatisfactory because the apertures of the light receiving elements cannot be reduced to a required level in order to ensure a sufficient quantity of light. With the conventional interferometers, it is common practice to measure a surface shape of a polished optical surface and it is difficult to measure a surface shape of a scattering surface.
Embodiments of the present invention have been accomplished in view of the above-described problems and it is an object of the present invention to provide an interferometer capable of measuring a surface shape of an examined object even if there is a large difference between surface shapes of the examined surface and a reference surface. It is another aspect of an embodiment of the present invention to provide an interferometer capable of measuring a surface shape of a scattering surface.